Covalent functionalization of MoS2 nanosheets synthesized by liquid phase exfoliation to construct electrochemical sensors for Cd (II) detection

Talanta 182 (2018) 38–48

Contents lists available at ScienceDirect

Talanta journal homepage: www.elsevier.com/locate/talanta

Covalent functionalization of MoS2 nanosheets synthesized by liquid phase exfoliation to construct electrochemical sensors for Cd (II) detection

T

⁎

Xiaorong Gana,b, Huimin Zhaoa, , Kwok-Yin Wongb, Dang Yuan Leic, Yaobin Zhanga, Xie Quana a

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China) School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, China c Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Surface functionalization Few-layer MoS2 nanosheets Liquid Phase exfoliation Electrochemical sensor Cd2+

Surface functionalization is an effective strategy in the precise control of electronic surface states of twodimensional materials for promoting their applications. In this study, based on the strong coordination interaction between the transition-metal centers and N atoms, the surface functionalization of few-layer MoS2 nanosheets was successfully prepared by liquid phase exfoliation method in N, N-dimethylformamide (DMF), 1-methyl-2-pyrrolidinone, and formamide. The cytotoxicity of surface-functionalized MoS2 nanosheets was for the first time evaluated by the methylthiazolyldiphenyl-tetrazoliumbromide assays. An electrochemical sensor was constructed based on glass carbon electrode (GCE) modified by MoS2 nanosheets obtained in DMF, which exhibits relatively higher sensitivity to Cd2+ detection and lower cytotoxicity against MCF-7 cells. The mechanisms of surface functionalization and selectively detecting Cd2+ were investigated by density functional theory calculations together with various spectroscopic measurements. It was found that surface-functionalized MoS2 nanosheets could be generated through Mo-N covalent bonds due to the orbital hybridization between the 5 s orbitals of Mo atoms and the 2p orbitals of N atoms of the solvent molecules. The high selectivity of the sensor is attributed to the coordination reaction between Cd2+ and O donor atoms of DMF adsorbed on MoS2 nanosheets. The robust anti-interference is ascribed to the strong binding energy of Cd2+ and O atoms of DMF. Under the optimum conditions, the electrochemical sensor exhibits highly sensitive and selective assaying of Cd2+ with a measured detection limit of 0.2 nM and a linear range from 2 nM to 20 μM.

1. Introduction Two-dimensional (2D) MoS2, as an emerging but important class of inorganic graphene analogs, exhibits unique electronic, optical, mechanical, and chemical properties [1,2]. 2D MoS2 possibly complements or even surpasses graphene (zero band gap) in electronics and optoelectronics fields due to various advantages such as intrinsic finite bandgap, high on/off ratio, and abundance of bulk material [3–6]. In general, the potential applications of 2D MoS2 hinge on the effective strategy for controlling the electronic surface states, which play a dominant role in affecting the various properties [7]. Surface modification could offer an alternative way to tune the properties of 2D MoS2 [8]. The surface modification of 2D MoS2 (e.g., the introduction of surface terminations such as hydroxyl, oxygen or fluorine) could not only minimize the drawbacks, such as high contact barrier between nanosheets, easy aggregation, and poor solubility/wettability in

⁎

common solvents [9], but also obtain new desirable functions to widen the potential applications [6]. Similar to other inorganic graphene analogs, 2D MoS2 has trigged intensive interest in electrochemical sensors due to the unique physical and chemical properties for electrochemical reactions [10], such as high active sites on the plane edges, and big specific surface areas for enlarging the electrode/electrolyte junction area. Especially the ultrathin plane structure where the electrons/holes are confined to a plane of atomic thickness, endows 2D MoS2 with sensitive surface states by quantum confinement effects [11,12]. Therefore, 2D MoS2 is a promising building block for constructing electrochemical sensors [13]. In general, the performances of 2D MoS2-based electrochemical sensors closely depend on the synthesis methods of the electrode materials [6]. Among various synthesis methods, liquid-phase exfoliation (LPE) is the most promising technique in terms of low-cost and mass production [1,14,15]. 2D MoS2 prepared by LPE could show the enhanced

Corresponding author. E-mail address: [email protected] (H. Zhao).

https://doi.org/10.1016/j.talanta.2018.01.059 Received 7 October 2017; Received in revised form 7 January 2018; Accepted 21 January 2018 0039-9140/ © 2018 Published by Elsevier B.V.

Talanta 182 (2018) 38–48

X. Gan et al.

electrocatalytic activity, which could further improve the sensitivity [16]. However, up to now, it has been rarely reported about exploring the possibility of the covalent functionalization of MoS2 nanosheets by LPE, and the influences of surface functional groups on the recognition performances towards analytes, especially heavy metal ions. Most of 2D MoS2-based electrochemical sensors were used only for the detection of organic small molecules or biological molecules [6]. Bearing this fact in mind, the subject of this study is to synthesize the covalently functionalized MoS2 nanosheets without changing the original lattice structure, because current methods for surface modifications, such as weak non-covalent interactions, covalent bonding at defects [9], and nanoparticle decoration, tend to be unstable or disturb the original lattice structure [7]. Surface-functionalized MoS2 nanosheets with high electrocatalytical activity and low cytotoxicity were further screened, and used to construct highly selective and sensitive electrochemical sensors for Cd2+, because the covalent attachment of functional groups could not only specifically recognize the heavy metal ions [17], but also facilitate electron transfer for improving the detection sensitivity [9]. Using density functional theory (DFT) calculations together with various spectroscopic measurements, the mechanisms of surface functionalization and selectively detecting Cd2+ were systematically investigated. This study would offer a new routine for developing 2D MoS2-based electrochemical sensors for detecting heavy metal ions.

(DMF, formamide, or NMP) was added as the exfoliation and dispersion solvent. The mixtures (1 mg/mL) were sonicated at a power output of 127.5 W (17% × 750 W) in a water-cooled bath at 25 °C for 12 h. The as-formed suspensions were centrifuged at 1000 rpm for 5 min to remove the residual bulk MoS2, and further centrifuged at 13,000 rpm for 10 min. Finally, the powders of few-layer MoS2 nanosheets were obtained by the vacuum freeze-drying. For simplicity, MoS2 nanosheets were synthesized via LPE in DMF, formamide, and NMP, accordingly labeled as MoS2-a, MoS2-b, and MoS2-c.

2. Experiments

2.5. Cell cultures and MTT assays

2.1. Reagents and solutions

MCF-7 cells were cultured in Dulbecco's Modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) at 37 °C in a 5% CO2/95% air incubator. The cells were seeded on 96-well cell culture plates (4×103 cells/well, 100 μL), and maintained for 24 h. Then, MoS2 nanosheets dispersed in 100 μL medium were added to the wells, and incubated for 24 h. Sequently, 20 μL of 5.0 mg/mL 3-(4,5-dimethylthiazol-2-yl)−2,5-diphenyltetrazolium bromide (MTT) was added, and cultured for more than 4 h. Then the medium was removed. Finally, 200 μL of dimethylsulfoxide was added to dissolve the formazan crystals. The optical density (OD) of MCF-7 cells was recorded by a Thermo Scientific Multiskan GO spectrophotometer at 570 nm.

2.4. Preparation of MoS2 nanosheet/GCE MoS2 nanosheets were dispersed in ultrapure water by ultrasonication for 10 min to generate a homogeneous suspension (1 mg/ mL). Prior to the electrode modification, glassy carbon electrode (GCE) was polished in sequence with alumina suspensions of 1, 0.3, and 0.05 µm in diameter, and each was sonicated in absolute ethanol and ultrapure water for 5 min to remove the possible bound microparticles. After that, the electrode was dried under nitrogen at room temperature. Finally, the clean GCE was modified by 10 μL of MoS2 nanosheet suspension (1 mg/mL), and directly dried in air to obtain MoS2 nanosheets modified GCE (MoS2 nanosheet/GCE).

Molybdenum sulfide (2H-MoS2, 99.5%) was purchased from Aladdin Industrial Corporation (Shanghai, China). N, N-dimethyl formamide (DMF), formamide, dimethylsulfoxide (DMSO), and 1-methyl2-pyrrolidinone (NMP) were bought from Tianjin Kemiou Chemical Reagent Co., Ltd., and were of analytical grade without additional purification. A human breast adenocarcinoma cell line (a widely studied epithelial cancer cell line derived from breast adenocarcinoma), named as MCF-7, and the cell growth medium (0.01 mg/mL bovine insulin, 10% fetal bovine serum, penicillin/streptomycin), named as MEM-Gibco 41500-034, were bought from Pro-cell Co., Ltd. All solutions in the experiments were prepared with ultrapure water (Milli-Q water, 18.2 MΩ cm) from a Millipore Milli-Q system (Bedford, MA, USA).

2.6. Computational details Periodic DFT calculations for the surface adsorption between (010) plane of (1 × 2 × 1 super cells) MoS2 and solvent molecules (DMF, formamide, and NMP) were performed using the CASTEP code under the general gradient approximation (GGA) expressed by the PerdewBurke-Ernzerhof (PBE) functional [18,19]. A vacuum thickness of 18 Å was used to avoid the coupling of interlayer. The kinetic energy cutoff and the convergence threshold for energy were set as 500 eV and 10−6 eV, respectively. All calculations for the mechanism of selectively detecting Cd2+ were performed using the DMol3 code. The nonlocal GGA functional by Perdew and Wang (PW91) was used for all geometry optimizations. A basis set of numeric atomic functions (DNP) has been used after considering the water solvent effect [20]. Gibbs free energy and enthalpy were corrected by the thermal energy at 298 K. In addition, the diffusion coefficients of Cu2+ and Ni2+ in pure water were calculated by molecular dynamics (MD) simulations. The force field, condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS), was chosen to predict the structures consisting of Cu2+ or Ni2+ and water molecules, where the amorphous cell is composed of 225 water molecules and one metal ion. The electrostatic forces were calculated by the Ewald summation method. A 12 Å cutoff was used for the short-range interactions and the real part of the Ewald sum [21]. The natural bond orbital (NBO) and the molecular electrostatic potential (ESP) analysis were calculated using Gaussian 09 program with B3LYP/6–311+G (d, p) [22].

2.2. Apparatus The morphologic characterizations were obtained by field emission scanning electron microscope (FE-SEM, S4800 Hitach), transmission electron microscopy (TEM, TecnaiG2S-Twin), and atomic force microscopy (AFM, Agilent PicoPlus). The chemical structure information was investigated by Fourier transform spectrophotometer (FT-IR, VERTEX 70, Bruker) with KBr as the reference sample. X-ray diffraction (XRD) patterns were performed on a Shimadzu Model LabX XRD-6000. Ultraviolet-visible spectrophotometry (UV–vis) absorption spectra were obtained on a Shimadzu Model UV-2450 spectrophotometer, and the surface charge was evaluated by a Zetasizer Nano ZS (Malvern Instrument Ltd, UK) in phosphate buffered solution (PBS) with pH = 7.4. The optical density (OD) of MCF-7 cells was recorded by a Thermo Scientific Multiskan GO spectrophotometer at 570 nm (630 nm was used as reference wavelength). Inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500, Agilent, USA) was applied for determination of the heavy metals in this work. 2.3. Preparation of MoS2 nanosheets Briefly, 30 mg of the as-prepared bulk MoS2 precursor was added into a 100 mL glass vial. Then, 30 mL of polar micromolecular solvent 39

Talanta 182 (2018) 38–48

X. Gan et al.

∆Hmix 2 ≈ (σsol-σNS)2∅ V TNS

2.7. Electrochemical measurements Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and square wave anodic stripping voltammetry (SWASV) were undertaken at room temperature by a CHI 660D electrochemical workstation (Shanghai Chenhua Limited, China). A conventional threeelectrode system was used. Thereinto, a bare GCE (diameter of 3 mm) or modified GCE was used as a working electrode, a platinum wire as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. Both CV and EIS experiments were performed in potassium ferricyanide solution containing 0.1 M KCl, 2.5 mM K3[Fe (CN)6], and 2.5 mM K4[Fe(CN)6]. SWASV measurements were employed to detect Cd2+ with or without other interfering metal ions. After optimizing the experimental parameters of SWASV, the reduction of metal ions (e.g., Cd2+ +2e→ Cd) was performed at −0.6 V (deposition potential) for 90 s (deposition time) in 0.1 M acetate buffer solution (NaAc-HAc) with pH = 7.0. In addition, the anodic stripping of the electrodeposited metal was carried out in the potential range from −1.0 to 0.5 V at the following parameters: frequency, 15 Hz; amplitude, 25 mV; increment potential, 4 mV. Of note, prior to the heavymetal ions detection, 1 mL of NaAc-HAc buffer solution was purged at least for 5 min with N2.

(1)

Where σsol and σNS are the total surface energies of solvent and MoS2 nanosheets, respectively, TNS and ∅ are the thickness and the volume fraction of MoS2 nanosheets, respectively. In addition to σ, the polar/ dispersive ratio (σp/σd) of efficient solvent is required to match with that of MoS2 nanosheets (σp/σd = 0.449) [26]. Therefore, the MoS2 dispersions obtained in DMF (Table S1), rather than in formamide or NMP, show the highest yield. After the optical absorption spectra of MoS2 dispersions were normalized to the local minimum (Fig. S1c) [27], both size and thickness of MoS2 nanosheets could be quantified as summarized in Fig. S1d. Thereinto, the thicknesses of MoS2-a, MoS2-b, and MoS2-c are equal to 9, 5, and 11 layers (one layer is equal to 6.5 Å), respectively, which are consistent with the experiment values (5.8, 3.2, and 7.5 nm) obtained by AFM measurements (Fig. S2). The phenomena demonstrate that DMF as the efficient exfoliation solvent could give the maximum dispersed concentration, but hardly makes MoS2 nanosheets fully exfoliated in terms of the thickness and size. Because the trace water present in solvent is crucial for the exfoliation efficiency (yield) and the size of MoS2 nanosheets [23,25,28], the mixtures of DMF and water with different volume ratios were used as the solvents for exfoliating MoS2 nanosheets. Previous report pointed out that the decrease of the amount of water in NMP could make MoS2 nanosheets highly fragmented, but lower the yield in turn [25]. In this study, the presence of trace water in DMF could obtain dispersions of MoS2 nanosheets at large concentrations (Fig. S3). However, the presence of trace water could to some extent promote the fragment of MoS2 nanosheets, which is different from the mixture system of NMP and water, possibly because the intercalation effects of solvent molecules can lead to swelling, and eventually influence delamination during LPE process [29]. In addition, the same trend could be observed in TEM images (Fig. 2), that is, both size and thickness of MoS2-b are relatively smaller than those of MoS2-a or MoS2-c. The phenomena suggest that the size and thickness of MoS2 nanosheets obtained by LPE is not conformed to the screening principle for the proper solvent that could only guarantee the exfoliation yield and stability of MoS2 nanosheets. For the exfoliation of MoS2 nanosheets, the intercalation process most likely occurs as the layered material is immersed into the organic nitrogen-containing solvent at a suitable temperature or vapor pressure [30]. The XRD patterns (Fig. S4) of MoS2-a and MoS2-b show that the peaks

3. Results and discussion 3.1. Morphology characterizations As shown in Fig. 1, the samples (MoS2-a, MoS2-b, and MoS2-c) obtained by LPE feature the typical morphology of 2D nanomaterials. Compared with bulk MoS2, both size and thickness of the samples are obviously decreased. From either the color of MoS2 nanosheet dispersions (digital images in Fig. S1a) or their intensity of optical absorption spectra (Fig. S1b), it could be concluded that the yield of MoS2 nanosheets exfoliated in DMF or NMP is much higher than that obtained in formamide. Generally, DMF or NMP is viewed as the most appropriate solvent for exfoliating bulk MoS2 [23–25], because the total surface tension (σ) of DMF or NMP is much closer to that of MoS2 nanosheets (σ = 46.5 mJ/m2) so that the energetic cost (described by Eq. (1)) of dispersing MoS2 nanosheets could be minimized [26], according to the principle about the enthalpy of mixing [15], ∆Hmix , per volume of mixture, V:

Fig. 1. SEM images of bulk MoS2 (a) and MoS2 nanosheets exfoliated in DMF (b), formamide (c), and NMP (d). Here, MoS2 nanosheets were synthesized via LPE in DMF, formamide, and NMP, accordingly labeled as MoS2-a, MoS2-b, and MoS2-c.

40

Talanta 182 (2018) 38–48

X. Gan et al.

Fig. 2. TEM, HRTME, and fast Fourier transform (FFT)-generated SAED patterns (from left to right) of MoS2 nanosheets obtained in DMF (a), formamide (b), and NMP (c). Inset: Measuring d-spacing and corresponding profile plot of the calibration. Here, MoS2 nanosheets were synthesized via LPE in DMF, formamide, and NMP, accordingly labeled as MoS2-a, MoS2-b, and MoS2-c.

dots, indicating of the good crystalline quality before and after LPE.

corresponding to (002) and (006) planes indeed exhibit a slight shift to lower 2θ values, indicating of a large expansion along c axis due to the solvent intercalation. Therefore, it is reasonable to believe that the effect of solvent intercalation leads to the discrepancies that thickness and size is not conformed to the screening principle for proper solvent [13,25]. In addition, compared with bulk MoS2 (JCPDS card no. 01073-1508, a = 3.1500 Å, c = 12.3000 Å), the peaks corresponding to (002) and (006) facets become widened and weak, possibly due to the decreased size. The P63/mmc unit cell could be observed before and after LPE process, suggesting that the solvent intercalation do not cause the phase transition (e.g., from 2H-MoS2 to 1T-MoS2), because the 1 T phase is unstable, and readily transforms back to the thermodynamically stable 2 H phase [6]. On the other hand, the mismatch in the surface tension between formamide and MoS2 crystals makes the resulting hydrodynamic forces most appear on the crystal edges, which is beneficial for the solvent intercalation between MoS2 sheets [31]. The resultant MoS2 nanosheets with the smallest size exhibit the thinnest thickness (Fig. S1d), and vice versa, again indicating the presence of the solvent intercalation, because the decreased lateral size could further increase the fraction of edges, which would promote the intercalation process of solvent molecules [31]. Smaller size, lower vapor pressure, and the planarity structure of formamide as a “molecular wedge’’ may enable the maximum number of solvent molecules to be packed between van der Waals gap of MoS2 sheets, and consequently make MoS2 nanosheets swell through the ultrasonication [30]. The HRTEM observations indicate that all MoS2 nanosheets display clearly crystal lattices. The measured d-spacing is 0.281 nm that is consistent with the (100) planes of 2H-MoS2. The corresponding fast Fourier transform (FFT)-generated SAED patterns also show the well-define diffraction

3.2. Characterizations and DFT calculations of the surface functionalization FT-IR spectra of MoS2-a, MoS2-b, and MoS2-c (Fig. 3A) show that the stretching vibration peaks of functional groups including C-N, C˭O, and C-H could be clearly observed. In addition, there were another two modes at 3159 and 3420 cm−1 from MoS2-b, corresponding to the stretching vibration peaks of -NH2. The phenomena imply that the functional groups on the surface of MoS2 nanosheets should be deprived from the corresponding solvents where MoS2 nanosheets were obtained by LPE. The XPS spectra (Fig. 3B) show that there are three peaks including Mo (3p3/2, 395.6 eV), Mo (3p1/2, 413.1 eV) and N (1 s, 399.8 eV), which are similar to those of nanoscale γ-Mo2N [32]. This proves that the surface functionalization of MoS2 nanosheets could take place by Mo-N covalent bonds during LPE process. Perfect MoS2 (with no defects) belongs to the dangling-bond-free atomic sheets (at basal plane); therefore, the covalent functionalization should form at some active sites (with high surface energy) such as the plane edges and site defects [33]. In order to verify our expectation, the Gibbs free energy (△G) of the adsorption process was evaluated by DFT calculations based on the model of the solvent molecules adsorbed on different crystalline planes including (001) and (010)/(100) planes of 2H-MoS2. Of note, the adsorption on (010) planes is equivalent to that on (100) planes for the same solvent molecule. As indicated in Fig. 3C, △G on (010) planes (labeled as △G(010)) are determined as −1.416 eV for DMF, −1.182 eV for formamide, and −2.416 eV for NMP, respectively, which are much stronger than △G on (001) planes (△G(001) = 41

Talanta 182 (2018) 38–48

X. Gan et al.

Fig. 3. (A) FT-IR spectra and (B) high-resolution X-ray photoelectron spectra of MoS2-a, MoS2-b, and MoS2-c. (C) The thermodynamically most stable conformations, the Gibbs free energies (△G), and the Mo-N bond lengths of the three complexes composed of solvent molecules and (1 × 2 × 1 super cells) 2H-MoS2. MoS2 nanosheets were exfoliated in DMF, formamide, and NMP, labeled as MoS2-a, MoS2-b, and MoS2-c, respectively.

Fig. 4. (A) Cytotoxicity assessment and (B) Zeta potentials of MoS2 nanosheets exfoliated in DMF (MoS2-a), formamide (MoS2-b), and NMP (MoS2-c), respectively.

−0.048, −0.06, and −0.25 eV for DMF, formamide, and NMP, respectively). These results are consistent with the reported adsorption energy (−0.09 eV) of -NH2 group on (001) planes of 2H-MoS2 [34]. Both XPS spectra and DFT calculations confirm that the chemical adsorption processes of formamide, DMF, and NMP molecules mainly take place on the (010) or (100) planes of 2H-MoS2 by the Mo-N covalent bonds, because these planes are full of the defect sites (e.g., sulfur vacancies), where Mo atoms are unbonded with S atoms [35]. In addition,

the interaction of the solvent molecules on (001) planes of 2H-MoS2 should be the weak physical adsorption. Concerning the thermodynamically most stable conformations of the three complexes composed of solvent molecules and (1×2×1 super cells) 2H-MoS2, the MoN bond lengths of MoS2-a, MoS2-b, and MoS2-c are calculated as 2.269, 2.305, and 2.284 Å, respectively, which are consistent with the reported range of 2.154–2.236 Å [36]. From the partial density of states (PDOS) of the three solvent molecules before and after the chemical adsorption 42

Talanta 182 (2018) 38–48

X. Gan et al.

and NMP, formamide as the exfoliation solvent could introduce -NH2 on the surface of MoS2 nanosheets, which might enhance the cytotoxicity effect [38].

on the plane edges of MoS2 nanosheets (Fig. S5 A-D), it was found the presence of Mo-N covalent bonds should be attributed to the orbital hybridization between the 5 s orbitals of Mo atoms and the 2p orbitals of N atoms of the three solvent molecules.

3.4. Mechanism of selectively detecting Cd2+ 3.3. Cytotoxicity assessment With respect to organic small molecules as the recognition elements (via the cooperative metal-ligand interaction), there is no unanimous conclusion about the mechanism of selectively detecting Cd2+, which were explained by the coordination between Cd2+ and oxygen or nitrogen donor atoms, or between Cd2+ and thiolate groups [17,41]. For Cd2+ detection by SWASV, one critical factor of influencing selectivity is the adsorption process between Cd2+ and the recognition element (Scheme S1). FT-IR and XPS spectra (Fig. 3) have demonstrated that the functional groups on the surface of MoS2 nanosheets are from the corresponding solvent molecules adsorbed by physical or chemical forces (defect sites). For the sake of simplicity, DMF molecules could be acted as the sensing element for Cd2+ recognition. To figure out the possible binding sites between Cd2+ and DMF, the natural bond orbital (NBO) analysis and the molecular electrostatic potential (ESP) of DMF were investigated. As depicted Fig. S6A, the NBO values of O and N atoms are more negative (−0.622 for O and −0.501 for N) than those of C or H atoms, indicating that O and N atoms have much stronger trend of nucleophilic or more donating tendency to Cd2+ [42]. Therefore, O and N atoms are the most likely potential binding sites for Cd2+ adsorption. In addition, ESP shows that in terms of the effects of the molecule's electrons, O atoms of DMF molecules are most dominant, again indicating that O atoms should be most likely susceptible to the electrophilic attack from Cd2+ (Fig. 5A). In this context, three possible structures of the complex (labeled as DMF-Cd2+) were optimized. As shown in Fig. 5B, the thermodynamically most stable conformation is formed by the coordination reaction between Cd2+ and O, because its

MoS2 nanosheets have been extensively applied in biosensors, bioimaging, drug delivery, tissue engineering, and cancer therapy [6]; however, the reports available were focused on the cytotoxicity of MoS2 nanosheets synthesized by the Li+-intercalation method [37], rather than by LPE. Considering the potential applications of MoS2 nanosheets synthesized by LPE, the biological nanotoxicity of MoS2 nanosheets was for the first time evaluated by methylthiazolyldiphenyl-tetrazoliumbromide (MTT) assays. In this study, the dose-dependent responses of MCF-7 cells to MoS2 nanosheets were carried out by in-vitro model. As shown in Fig. 4A, the apoptosis rates of MCF-7 cells treated for 24 h in 500 μg/mL of MoS2-a, MoS2-b, and MoS2-c are 54.7%, 46.8%, and 30.5%, respectively. Generally, prolonging the exposure time of cells, or increasing the concentration of MoS2 nanosheets could cause more obvious apoptosis. For MoS2-b, the relatively strong apoptosis should be attributed to the smaller size and thickness (well-exfoliated MoS2 nanosheets), because the exfoliation extent of MoS2 nanosheets is one of main factors in affecting the cytotoxicity [37]. In addition to the nature of nanomaterials, the surface properties (e.g. hydrophobicity, charge and functional groups) could affect the cytotoxicity [38–40]. Therefore, the surface charges of MoS2 nanosheets were investigated. As demonstrated in Fig. 4B, the Zeta potentials of MoS2-a, MoS2-b, and MoS2-c were measured as −10.3 ± 0.3, −16.0 ± 0.6, and −8.02 ± 0.3 mV, respectively. The result demonstrates that MoS2-b could result in the strong phagocytosis process possibly due to the relatively high surface charge. Compared with DMF

Fig. 5. (A) Molecular electrostatic potential surface (ESP) of DMF obtained through DFT calculations based on B3LYP/6–311 + G (d, p). (B) Possible structures, optimized structure parameters, and total energies of the complex composed of DMF and Cd2+. (C) The plot of Gibbs free energy vs. the number of H2O. (D) FT-IR spectra of MoS2-a before (spectrum a) and after (spectrum b) detecting 5 μM Cd2+.

43

Talanta 182 (2018) 38–48

X. Gan et al.

formed by electrochemical deposition of Cd2+ (Cd2+ + 2e → Cd). From DFT calculations and experiments, the recognition process of the sensing element is derived from the coordination interaction between the carbonyl group and the hydrated Cd2+ [45].

total energy (−5716.06 Ha) is lowest than other conformations generated by Cd2+-N (−5716.04 Ha) and O-Cd2+-N (−5715.74 Ha) coordination interactions. Considering the ion hydration that was required for Cd2+ detection, the influences of the number of water on the stability of the complex, [(DMF)Cd(H2O)6−n]2+ (n = 0, 1, 2, 3, 4, and 5), were further investigated. Because the average hydration number of Cd2+ is six [43], the coordination reaction should be

3.5. Electrocatalytical activity and specificity

DMF + [Cd(H2 O)6]2 + ↔ [(DMF)Cd(H2 O)6 − n]2 + + nH2 O

The electrocatalytical activity of MoS2 nanosheets modified GCEs and the electrode specificity to certain heavy metal ion were investigated. As shown in Fig. 6A, the voltammogram shows two irreversible redox peaks at potentials of ~0.1 and ~0.2 V versus SCE, respectively. Compared with bulk MoS2-modified GCE, MoS2-a/GCE, MoS2-b/GCE, and MoS2-c/GCE show the sharp increase in the redox peak currents (Ip), indicating that the catalytic performance of MoS2 nanosheets are improved, possibly because of the increased edge active sites [46], the enhanced hydrophily and conductivity after the surface functionalization [47,48]. Similar to other layered nanomaterials [49], the electron injection from the formamide molecules into the framework of MoS2 could enhance the density of electronic states of MoS2 near the Fermi level, which could greatly facilitate the interfacial electron transfer between the redox species (e.g., ferricyanide) and MoS2 nanosheets [50]. To gain better insight into the interface properties of electrodes, EIS was employed to further examine the detailed information on the impedance changes of MoS2 nanosheets-modified GCEs. As shown in Fig. 6B, the EIS of each modified GCE exhibits a semicircle over the high frequency range, followed by a straight 45°

Consequently, the net change of Gibbs free energy (△G) could be calculated by

∇Gcomplex = ∇G[(DMF ) Cd (H2 O)6 − n]2 + + n∇G H2 O − ∇GDMF − ∇G[Cd (H2 O)6]2 + For different numbers of water, △G could be calculated as shown in Fig. 5C. As n = 3, △G became negative, indicating that the recognition process between DMF and hydrated Cd2+ could spontaneously proceed (thermodynamically favorable). As such, the most stable conformation of the complex is [(DMF)Cd(H2O)3]2+ (Fig. S6B), whose structure parameters were summed in Table S2. To further confirm the conclusion from DFT calculations, FT-IR spectra (Fig. 5D) were used to test the changes of electrode surface before and after assaying 5 μM Cd2+. Compared with pristine MoS2 nanosheets, the peak of the carbonyl group stretching vibration (νC˭O) disappeared after detecting Cd2+, indicating that the coordination interaction took place between Cd2+ and O atom of the carbonyl group [44]. The conclusion was confirmed by the presence of νO-H, which should be from the strong adsorption and dissociation of water molecules (H2O + Cd→ Cd-OH + Cd-H) on Cd

Fig. 6. Cyclic voltammograms (A) and EIS (B) of GCE and modified GCEs in potassium ferricyanide solution containing 0.1 M KCl, 2.5 mM K3[Fe(CN)6], and 2.5 mM K4[Fe(CN)6] with a scan rate of 50 mV s−1. The electron transfer resistance (Rct) is equals to the semicircle diameters. The inset shows equivalent electrical circuit diagrams, where RE is the electrolyte resistance, Cd the double-layer capacitance, RCT an electron-transfer resistance, and ZW the Warburg impedance. (C) SWASV detection plots of 0.1 μM Ni2+, Cu2+, Pb2+, Cd2+ and Hg2+ on MoS2 nanosheets- modified GCEs. MoS2-a, MoS2-b, and MoS2-c represent MoS2 nanosheets synthesized via LPE in DMF, formamide, and NMP, respectively.

44

Talanta 182 (2018) 38–48

X. Gan et al.

Fig. 7. Optimization of experimental conditions including supporting electrolytes (A), pH value (B), deposition potential (C), and deposition time (D) of the SWASV responses on MoS2-a/ GCE to 0.1 μM Cd2+. Error bars represent the standard deviation across three repetitive experiments using three different electrodes.

significantly affect the sensing performances. In the study, SWASV was chosen as the analytical technique to acquire the qualitative information, because it combines the intrinsic advantages of ASV and SWV [50–52]. There are two typical steps (Scheme S1) using the SWASV technique to detect heavy metal ions: (1) the deposition of heavy metal ion(s) at an optimized potential for a certain time duration, and (2) the anodic stripping of the deposited heavy metal. The anodic stripping signal is used to monitor the concentration of Cd2+ in solution. Accordingly, four parameters of SWASV including the supporting electrolyte, pH value, deposition potential, and deposition time were optimized based on the results of detecting 100 nM Cd2+. As indicated in Fig. 7A, the sensitivity to Cd2+ detection is dependent on the four different supporting electrolytes including 0.1 M NH4Cl-HCl, a phosphate buffer, an acetate buffer, and a citric/sodium citrate buffer. When acetate buffer was used as the supporting electrolyte (pH = 7), MoS2-a/ GCE showed a relatively high stripping current. Therefore, 0.1 M acetate buffer solution was chosen as the supporting electrolyte in the following experiments. In addition, the pH value of supporting electrolyte could affect the interfacial electrochemical reactions. Therefore, the independence of the sensing performance on the pH value of acetate buffer solution was further investigated. As shown in Fig. 7B, the stripping peak current was increased within pH 3.0–4.0, but slightly decreased at pH = 5.0, and then gradually increased within pH 5.0–7.0. Overall, too low pH may be not beneficial for the stability of organic molecules [50,53]. In this sense, low-pH buffer may influence the immobilization of DMF on the surface of MoS2 nanosheets; therefore, 0.1 M acetate buffer solution (pH = 7.0) is selected in the following experiments. Similarly, the deposition potential (Fig. 7C) and the deposition time (Fig. 7D) were optimized to be −0.6 V and 90 s, respectively. Of note, 90 s as the best deposition time was used to avoid the saturation response at higher concentration of Cd, because

sloped line in the low frequency region. Thereinto, the semicircular portion corresponds to the electron-transfer-limited process, and the electron transfer resistance (Rct) is equals to the semicircle diameter [49]. Rct of bulk MoS2/GCE, MoS2-a/GCE, MoS2-b/GCE, and MoS2-c/ GCE are 3406, 855, 676, and 1083 Ω, indicating that the heterogeneous electron transfer resistance is decreased after surface functionalization. The specificity test (Fig. 6C) demonstrates that MoS2-a/GCE, MoS2-b/ GCE, and MoS2-c/GCE all show highly selective responses to Cd2+ detection, because no electrochemical signal of Cu2+, Ni2+, Pb2+, or Hg2+ could be observed. Furthermore, compared with MoS2-b/GCE and MoS2-c/GCE, MoS2-a/GCE exhibits a well-defined current peak of Cd2+, indicating that MoS2-a/GCE would possess robust anti-interference ability and sensitivity to Cd2+ detection. Considering the electrocatalytic activity and cytotoxicity as well as the sensitivity for Cd2+ detection, MoS2-a/GCE was used to construct the electrochemical sensor. Furthermore, DFT calculations were used to investigate the binding energies of DMF and hydrated Cd2+, Cu2+, Ni2+, Pb2+, or Hg2+ (Table S3). It was found that among the five typical heavy metal ions, the strongest binding energy is formed between hydrated Cd2+ and DMF, and the next is between hydrated Ni2+ and DMF. Both Cu2+ and Cd2+ belong to the borderline acids, and thus tend to bind with ligands containing N donor atoms [17]. However, the steric effects and the different environments of N atoms such as -NH2, -NH-, and –N = would also affect bond strengths or binding energies between heavy metal ions and the sensing element [50,53], which could result in the apparent changes of electrical signal response (compete absorption) [41]. 3.6. Optimization of the experimental conditions for Cd2+ Detection In addition to electrode materials, the assaying method could 45

Talanta 182 (2018) 38–48

X. Gan et al.

Fig. 8. (A) SWASV responses of the MoS2-a/GCE in 0.1 M acetate buffer solution (pH = 7.0) containing Cd2+(concentrations of 0.2 nM (trace a), 2 nM (trace b), 20 nM (trace c), 100 nM (trace d), 500 nM (trace e), 2 μM (trace f), 5 μM (trace g), 10 μM (trace h), 20 μM (trace i), and 50 μM (trace j). (B) Linear relationship between the peak currents and the logarithm of Cd2+ concentrations (2 nM~10 μM). Inset: plot of SWASV peak current vs. Cd2+ concentration. Error bars represent the standard deviation across three repetitive experiments using three different electrodes.

3.9. Analysis of Cd2+in real water samples

increasing the deposition time could improve the sensitivity, but reduce the upper detection limit [52].

In order to demonstrate the capability of the sensor for Cd2+ determination in environmental matrixes, the real water samples were collected from Ling Shui River (Dalian, China) and tap water from laboratory water pipe, and were used for SWASV measurements. The samples from Ling Shui River were filtered through a 0.2 µm membrane to remove the insolubles before Cd2+ detection. The concentration of

2+

3.7. Quantitative determination of Cd

Under the optimal experimental conditions, the sensitivity of the electrochemical sensor based on MoS2-a/GCE was investigated by the quantitative detection of Cd2+. Fig. 8A depicts SWASV responses of the MoS2-a/GCE in 0.1 M acetate buffer solution (pH = 7.0) containing Cd2+ with the concentration range from 0.2 nM to 50 μM (traces a-j). The peak current at −0.76 V vs. SCE clearly gradually becomes large with the increase of Cd2+ concentration, and the relationship between the peak current and the concentration of Cd2+ is plotted in the inset plot of Fig. 8B. A linear range (Fig. 8B) is obtained from 2 nM to 20 μM with a linear correlation of 0.990. The regression Equation is

I = 5.926 + 8.368 log

[CCd2+/nM ] (R2 = 0.990)

The measured detection limit is as low as 0.2 nM (> 3 S/N, S and N represent standard deviation and slope rate, respectively). Compared with most of the reported electrochemical sensors (Table S4), the detection limit and the linear range of the sensor show promising advantages. Of note, the detection limit is much lower than that stipulated in the Environmental Protection Agency (5 ppb) or the World Health Organization (3 ppb) guidelines about maximum contaminant level (MCL) for Cd2+ in drinking water [54]. Fig. 9. Selectivity investigation of the proposed sensor for Cd2+ detection. The data were evaluated from SWASV. The concentration of interfering metal ions was 5 μM. Error bars represent the standard deviation across three repetitive experiments using three different electrodes.

3.8. Selectivity investigation To explore the selectivity, the influences of possible interfering metal ions (Hg2+, Pb2+, Ni2+, Ca2+, Fe3+, Cu2+, Co2+, and Mg2+) on Cd2+ detection were investigated under the identical experiment conditions. As shown in Fig. 9, the SWASV peak current of 5 μM Cd2+ on MoS2-a/GCE is much stronger than any other metal ions at the same concentration. Of note, among these interfering metal ions, the electrical signal intensity of Cu2+ or Ni2+ is relatively much stronger than that of interfering metal ions, possibly due to the larger binding energy between the recognition element (DMF) and Cu2+ or Ni2+ (Table S3). Even though the binding energy between Cu2+ and DMF is smaller than that between Ni2+ and DMF, the diffusion coefficient of Cu2+ is 2.22 times larger than that of Ni2+ (Fig. S7), which may result in the more obvious interfering signal. All these selective experimental results indicate that the proposed electrochemical sensor exhibits the high specificity to Cd2+ and its sensitivity can meet the selective requirements for real environmental applications.

Table 1 Determination of Cd2+ in real water samples using MoS2-a/GCE. Each result is obtained by three repetitive experiments using three different electrodes. Sample No.

Tap1 Tap2 Tap3 Tap4 Tap5 Ling Shui River 1 Ling Shui River 2

46

[Cd2+](nM)

Recovery/%

ICP-MS

Spiked

Found

495 194 92 46 24 43 91

500 200 100 50 25 0 50

532 218 106 54 28 51 99

107 112 115 117 116 119 109

Talanta 182 (2018) 38–48

X. Gan et al.

Cd2+ in tap water is lower than the detection limit of ICP-MS; therefore, the spike-and-recovery method was used in this study. As shown in Table 1, the recovery is determined as 107–119%, indicating that the electrochemical sensor has a good reliability for Cd2+ determination. Recovery of Cd2+ on the MoS2-a/GCE is 91%, even though 100 nM Cd2+, Cu2+, Ni2+, Pb2+, and Hg2+ coexist in the buffer solution (Fig. 6C). Of note, the sample from Ling Shui River From is the natural water and contains various heavy metal ions (e.g., 24 nM Cu2+ and 100 nM Hg2+) and organic compounds; therefore, it is reasonable to conclude that the electrochemical sensor might have potential applications in environmental monitoring of Cd2+ exposure.

[13] S. Su, J. Chao, D. Pan, L.H. Wang, C.H. Fan, Electrochemical sensors using twodimensional layered nanomaterials, Electroanal 27 (2015) 1062–1072. [14] J.N. Coleman, M. Lotya, A. O'Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J.C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Two-dimensional nanosheets produced by liquid exfoliation of layered materials, Science 331 (2011) 568–571. [15] G. Cunningham, M. Lotya, C.S. Cucinotta, S. Sanvito, S.D. Bergin, R. Menzel, M.S.P. Shaffer, J.N. Coleman, Solvent exfoliation of transition metal dichalcogenides: dispersibility of exfoliated nanosheets varies only weakly between compounds, ACS Nano 6 (2012) 3468–3480. [16] X. Wang, F. Nan, J. Zhao, T. Yang, T. Ge, K. Jiao, A label-free ultrasensitive electrochemical DNA Sensor based on thin-layer MoS2 nanosheets with high electrochemical activity, Biosens. Bioelectron. 64 (2015) 386–391. [17] X. Wang, F. Nan, J. Zhao, T. Yang, T. Ge, K. Jiao, Electrochemical sensing of heavy metal ions with inorganic, organic and bio-materials, Biosens. Bioelectron. 63 (2015) 276–286. [18] S.J. Clark, M.D. Segall, C.J. Pickard, P.J. Hasnip, M.J. Probert, K. Refson, M.C. Payne, First principles methods using CASTEP, Z. Krist. 220 (2005) 567–570. [19] S.L. Zhang, Z. Yan, Y.F. Li, Z.F. Chen, H.B. Zeng, Atomically thin arsenene and antimonene : semimetal-semiconductor and indirect-direct band-gap transitions, Angew. Chem. Int. Ed. 54 (2015) 3112–3115. [20] F.T.I. Marx, J.H.L. Jordaan, G. Lachmann, H.C.M. Vosloo, Comparison of low and high activity precatalysts: do the calculated energy barriers during the self-metathesis reaction of 1-octene correlate with the precatalyst metathesis activity? J. Comput. Chem. 35 (2014) 1464–1471. [21] S. Kerisit, M. Vijayakumar, K.S. Han, K.T. Mueller, Solvation structure and transport properties of alkali cations in dimethyl sulfoxide under exogenous static electric fields, J. Chem. Phys. (2015) 142. [22] M.J.T. Frisch, W. G, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. [23] A. Jawaid, D. Nepal, K. Park, M. Jespersen, A. Qualley, P. Mirau, L.F. Drummy, R.A. Vaia, Mechanism for liquid phase exfoliation of MoS2, Chem. Mater. 28 (2016) 337–348. [24] F. Ghasemi, S. Mohajerzadeh, Sequential solvent exchange method for controlled exfoliation of MoS2 suitable for phototransistor fabrication, ACS Appl. Mater. Inter. 8 (2016) 31179–31191. [25] A. Gupta, V. Arunachalam, S. Vasudevan, Liquid-phase exfoliation of MoS2 nanosheets: the critical role of trace water, J. Phys. Chem. Lett. 7 (2016) 4884–4890. [26] J.F. Shen, Y.M. He, J.J. Wu, C.T. Gao, K. Keyshar, X. Zhang, Y.C. Yang, M.X. Ye, R. Vajtai, J. Lou, P.M. Ajayan, Liquid phase exfoliation of two-dimensional materials by directly probing and matching surface tension components, Nano Lett. 15 (2015) 5449–5454. [27] C. Backes, R.J. Smith, N. McEvoy, N.C. Berner, D. McCloskey, H.C. Nerl, A. O'Neill, P.J. King, T. Higgins, D. Hanlon, N. Scheuschner, J. Maultzsch, L. Houben, G.S. Duesberg, J.F. Donegan, V. Nicolosi, J.N. Coleman, Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets, Nat. Commun. 5 (2014) 1–10. [28] K. Manna, H.N. Huang, W.T. Li, Y.H. Ho, W.H. Chiang, Toward understanding the efficient exfoliation of layered materials by water-assisted cosolvent liquid-phase exfoliation, Chem. Mater. 28 (2016) 7586–7593. [29] P.L. Cullen, K.M. Cox, M.K. Bin Subhan, L. Picco, O.D. Payton, D.J. Buckley, T.S. Miller, S.A. Hodge, N.T. Skipper, V. Tileli, C.A. Howard, Ionic solutions of twodimensional materials, Nat. Chem. 9 (2017) 244–249. [30] R.H. Friend, A.D. Yoffe, Electronic-properties of intercalation complexes of the transition-metal dichalcogenides, Adv. Phys. 36 (1987) 1–94. [31] X. Hai, K. Chang, H. Pang, M. Li, P. Li, H. Liu, L. Shi, J. Ye, Engineering the edges of MoS2 (WS2) crystals for direct exfoliation into monolayers in polar micromolecular solvents, J. Am. Chem. Soc. 138 (2016) 14962–14969. [32] H.T. Chiu, S.H. Chuang, G.H. Lee, S.M. Peng, Low-temperature solution route to molybdenum nitride, Adv. Mater. 10 (1998) 1475–1479. [33] Q. Ding, K.J. Czech, Y.Z. Zhao, J.Y. Zhai, R.J. Hamers, J.C. Wright, S. Jin, Basalplane ligand functionalization on semiconducting 2H-MoS2 monolayers, ACS Appl. Mater. Inter 9 (2017) 12734–12742. [34] Q. Tang, D.E. Jiang, Stabilization and band-gap tuning of the 1T-MoS2 monolayer by covalent functionalization, Chem. Mater. 27 (2015) 3743–3748. [35] Z. Yu, Y. Pan, Y. Shen, Z. Wang, Z.Y. Ong, T. Xu, R. Xin, L. Pan, B. Wang, L. Sun, J. Wang, G. Zhang, Y.W. Zhang, Y. Shi, X. Wang, Towards intrinsic charge transport in monolayer molybdenum disulfide by defect and interface engineering, Nat. Commun. 5 (2014) 5290. [36] H.X. Li, M. Huang, G.Y. Cao, Markedly different adsorption behaviors of gas molecules on defective monolayer MoS2: a first-principles study, Phys. Chem. Chem. Phys. 18 (2016) 15110–15117.

4. Conclusions In summary, surface-functionalized MoS2 nanosheets by LPE in DMF, formamide, and NMP were successfully synthesized. It was found that the surface functionalization of MoS2 nanosheets was generated by Mo-N covalent bonds owing to the orbital hybridization between the 5 s orbitals of Mo atoms and the 2p orbitals of N atoms of solvent molecules. MoS2 nanosheets obtained in formamide exhibits smaller size and thickness, and higher cytotoxicity. Using MoS2 nanosheets exfoliated in DMF as the electrode material to construct an electrochemical sensor shows good specificity and sensitivity to Cd2+ detection. DFT calculations and various spectroscopic measurements demonstrate that the high selectivity is ascribed to the larger binding energy between Cd2+ and O atom of DMF. Additionally, the developed method has been demonstrated to be suitable for monitoring Cd2+ in real samples. Acknowledgments This study was supported by National Natural Science Foundation of China (No. 21477012) and the Programme of Introducing Talents of Discipline to Universities (B13012). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2018.01.059. References [1] C. Backes, T.M. Higgins, A. Kelly, C. Boland, A. Harvey, D. Hanlon, J.N. Coleman, Guidelines for exfoliation, characterization and processing of layered materials produced by liquid exfoliation, Chem. Mater. 29 (2017) 243–255. [2] C. Lee, H. Yan, L.E. Brus, T.F. Heinz, J. Hone, S. Ryu, Anomalous lattice vibrations of single- and few-layer MoS2, ACS Nano 4 (2010) 2695–2700. [3] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [4] R.S. Ningthoujam, N.S. Gajbhiye, Synthesis, electron transport properties of transition metal nitrides and applications, Progress. Mater. Sci. 70 (2015) 50–154. [5] S.Z. Butler, S.M. Hollen, L.Y. Cao, Y. Cui, J.A. Gupta, H.R. Gutierrez, T.F. Heinz, S.S. Hong, J.X. Huang, A.F. Ismach, E. Johnston-Halperin, M. Kuno, V.V. Plashnitsa, R.D. Robinson, R.S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M.G. Spencer, M. Terrones, W. Windl, J.E. Goldberger, Progress, challenges, and opportunities in two-dimensional materials beyond graphene, ACS Nano 7 (2013) 2898–2926. [6] X. Gan, H. Zhao, X. Quan, Two-dimensional MoS2: a promising building block for biosensors, Biosens. Bioelectron. 89 (2017) 56–71. [7] S.D. Lei, X.F. Wang, B. Li, J.H. Kang, Y.M. He, A. George, L.H. Ge, Y.J. Gong, P. Dong, Z.H. Jin, G. Brunetto, W.B. Chen, Z.T. Lin, R. Baines, D.S. Galvao, J. Lou, E. Barrera, K. Banerjee, R. Vajtai, P. Ajayan, Surface functionalization of two-dimensional metal chalcogenides by Lewis acid-base chemistry, Nat. Nanotechnol. 11 (2016) 465–471. [8] S. Presolski, M. Pumera, Covalent functionalization of MoS2, Mater. Today 19 (2016) 140–145. [9] D. Voiry, A. Goswami, R. Kappera, C.D.C.E. Silva, D. Kaplan, T. Fujita, M.W. Chen, T. Asefa, M. Chhowalla, Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering, Nat. Chem. 7 (2015) 45–49. [10] X.K. Kong, Q.C. Liu, C.L. Zhang, Z.M. Peng, Q.W. Chen, Elemental two-dimensional nanosheets beyond graphene, Chem. Soc. Rev. 46 (2017) 2127–2157. [11] Y.F. Sun, S. Gao, F.C. Lei, Y. Xie, Atomically-thin two-dimensional sheets for understanding active sites in catalysis, Chem. Soc. Rev. 44 (2015) 623–636. [12] F. Chen, J.L. Xia, D.K. Ferry, N.J. Tao, Dielectric screening enhanced performance in graphene FET, Nano Lett. 9 (2009) 2571–2574.

47

Talanta 182 (2018) 38–48

X. Gan et al.

[37] E.L.K. Chng, Z. Sofer, M. Pumera, MoS2 exhibits stronger toxicity with increased exfoliation, Nanoscale 6 (2014) 14412–14418. [38] Y.X. Liu, W. Li, F. Lao, Y. Liu, L.M. Wang, R. Bai, Y.L. Zhao, C.Y. Chen, Intracellular dynamics of cationic and anionic polystyrene nanoparticles without direct interaction with mitotic spindle and chromosomes, Biomaterials 32 (2011) 8291–8303. [39] S. Sharifi, S. Behzadi, S. Laurent, M.L. Forrest, P. Stroeve, M. Mahmoudi, Toxicity of nanomaterials, Chem. Soc. Rev. 41 (2012) 2323–2343. [40] A. Dhawan, V. Sharma, Toxicity assessment of nanomaterials: methods and challenges, Anal. Bioanal. Chem. 398 (2010) 589–605. [41] M. Remelli, V.M. Nurchi, J.I. Lachowicz, S. Medici, M.A. Zoroddu, M. Peana, Competition between Cd (II) and other divalent transition metal ions during complex formation with amino acids, peptides, and chelating agents, Coord. Chem. Rev. 327 (2016) 55–69. [42] P. Sjoberg, P. Politzer, Use of the electrostatic potential at the molecular-surface to interpret and predict nucleophilic processes, J. Phys. Chem. 94 (1990) 3959–3961. [43] Y. Marcus, Ions in Solution and their Solvation, Wiley, Hoboken, N J, 2015. [44] W.B. Bai, L.R. Fan, Y. Zhou, Y. Zhang, J.Y. Shi, G.H. Lv, Y.F. Wu, Q. Liu, J.Q. Song, Removal of Cd2+ ions from aqueous solution using cassava starch-based superabsorbent polymers, J. Appl. Polym. Sci. 134 (2017). [45] M.A. Henderson, The interaction of water with solid surfaces: fundamental aspects revisited, Surf. Sci. Rep. 46 (2002) 1–308. [46] J. Kibsgaard, Z.B. Chen, B.N. Reinecke, T.F. Jaramillo, Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis, Nat. Mater. 11 (2012) 963–969. [47] C.L. Wan, X.K. Gu, F. Dang, T. Itoh, Y.F. Wang, H. Sasaki, M. Kondo, K. Koga, K. Yabuki, G.J. Snyder, R.G. Yang, K. Koumoto, Flexible n-type thermoelectric

[48]

[49]

[50]

[51] [52]

[53]

[54]

48

materials by organic intercalation of layered transition metal dichalcogenide TiS2, Nat. Mater. 14 (2015) 622–627. C.W. Lin, X.J. Zhu, J. Feng, C.Z. Wu, S.L. Hu, J. Peng, Y.Q. Guo, L.L. Peng, J.Y. Zhao, J.L. Huang, J.L. Yang, Y. Xie, Hydrogen-incorporated TiS2 ultrathin nanosheets with ultrahigh conductivity for stamp-transferrable electrodes, J. Am. Chem. Soc. 135 (2013) 5144–5151. D.M. Adams, L. Brus, C.E.D. Chidsey, S. Creager, C. Creutz, C.R. Kagan, P.V. Kamat, M. Lieberman, S. Lindsay, R.A. Marcus, R.M. Metzger, M.E. Michel-Beyerle, J.R. Miller, M.D. Newton, D.R. Rolison, O. Sankey, K.S. Schanze, J. Yardley, X.Y. Zhu, Charge transfer on the nanoscale: current status, J. Phys. Chem. B 107 (2003) 6668–6697. X.R. Gan, H.M. Zhao, S. Chen, H.T. Yu, X. Quan, Three-dimensional porous HxTiS2 nanosheet-polyaniline nanocomposite electrodes for directly detecting trace Cu (II) Ions, Anal. Chem. 87 (2015) 5605–5613. A.C. Chen, B. Shah, Electrochemical sensing and biosensing based on square wave voltammetry, Anal. Methods-Uk 5 (2013) 2158–2173. Y. Wei, C. Gao, F.L. Meng, H.H. Li, L. Wang, J.H. Liu, X.J. Huang, SnO2/reduced graphene oxide nanocomposite for the simultaneous electrochemical detection of cadmium (II), lead (II), copper (II), and mercury (II): an interesting favorable mutual interference, J. Phys. Chem. C. 116 (2012) 1034–1041. X.R. Gan, H.M. Zhao, X. Quan, Y.B. Zhang, An electrochemical sensor based on paminothiophenol/Au nanoparticle-decorated HxTiS2 nanosheets for specific detection of picomolar Cu (II), Electrochim. Acta 190 (2016) 480–489. X.L. Chai, L.M. Zhang, Y. Tian, Ratiometric electrochemical sensor for selective monitoring of cadmium ions using biomolecular recognition, Anal. Chem. 86 (2014) 10668–10673.